1. Field of the Invention
The present invention relates to a position control device, adapted for use in a motor, particularly a vibration wave motor, employed as an actuator in robots for factory automation and in measuring instruments.
2. Related Background Art
Among the motors of various types there is known, in addition to the electromagnetic motors widely used, a vibration type motor utilizing the relative movement between a vibration member in which a vibration is generated by an electromechanical converting element and a contact member maintained in contact with the vibration member. Such vibration type motor is being employed as a position controlling actuator for a moving member in robots and in various measuring instruments, because of various reasons such as the excellent stability of rotation at a low speed.
There are already proposed position control devices utilizing such vibration type motor, such as the one which is proposed in the Japanese Patent Laid-open Application No. 63-18974 and in which the power supply to the motor is automatically turned off when the actual position (detected by a position sensor) of the movable member reaches a target position, or the one which is proposed in the Japanese Patent Laid-open Application No. 63-167681 and in which the power supply to the motor is turned off when the actual position reaches a position slightly in front of the target position, in anticipation of an over-run, thereby stopping the movable member at the target position, or the one which is proposed in the Japanese Patent Laid-open Application No. 63-209478 and in which an inverse voltage is applied to the motor to rapidly stop the motor and the movable member when the actual position reaches the target position or the vicinity thereof.
In such devices, when the actual position of the movable member does not reach the target or overruns the target position when the motor is stopped, the motor is repeatedly re-activated until the actual position of the movable member coincides with the target position or the control is terminated without such corrective actions.
However, in the configuration in which the power supply to the motor is turned off when the actual position of the movable member coincides with the target position or a position in front thereof, the motor control setting has to be executable exactly when the actual position coincides with the target or front position, regardless of the function timing of the control device, since otherwise the actual position may overrun the target or front position between the function timings so that the stopping control may become meaningless.
Also in the configuration of rapidly stopping the movable member, it causes vibration by the impact generated by the rapid deceleration of the movable member whereby the detected position of the movable member vibrates around the actual stopping position. As a result, the movable member is judged as out of the target position and the motor is re-started. Consequently there may be required a long time before the motor is completely stopped or caused to generate a reciprocating motion.
Also when an external vibration is applied to the stopped movable member, even if there is scarce relative movement between the vibration member and the contact member of the vibration type motor, the detected position of the movable member vibration and may be judged as out of the target position, whereby the motor may be re-started.
Furthermore, if the overrun of the movable member is significant, reciprocating motions may continue without stopping at the target position.
In another position controlling method, the actual position of the movable member is not immediately brought to the final target position, but there are selected intermediate target positions successively approaching the final target position and the control is so conducted as to make the actual position of the movable member to coincide with such intermediate target positions in succession. In such control method, however, the movement of the movable member to the final target position is not smooth, i.e., if the motor is stopped upon reaching each intermediate target position.
Furthermore, if the drive means is driven with a high speed, the arrival of the actual position at the target position may be overlooked.